The present invention relates to a film bulk acoustic resonator that can be suitably used for a small-sized high-frequency filter to be used in a communication device.
In recent years, due to improvements in functions and speeding up of communication devices, such as cellular phones and PDA (personal digital assistant: personal portable information communication) devices, further miniaturization and reduction in cost have been required for internal high-frequency filters operating at a frequency from several hundred MHz to several GHz. As promising candidates that meet such requirements, band-pass filters in which film bulk acoustic resonators (Film Bulk Acoustic Resonators: FBARs) are arranged in a ladder form and through which only electric signals in a desired frequency band pass with low loss are available. This is because, since film bulk acoustic resonators are formed by application of a semiconductor manufacturing technology, reductions in size and cost can be easily achieved.
FIG. 7 shows, as an example of a typical configuration of a film bulk acoustic resonator, a configuration of a film bulk acoustic resonator of an air-bridge type. Here, part (a) of FIG. 7 is a plan view, and parts (b) and (c) of FIG. 7 are sectional views taken along line V-V′ and line I-I′, respectively, of part (a) of FIG. 7. In the film bulk acoustic resonator shown in the drawings, a resonant portion A formed by stacking a first electrode 2, a piezoelectric material layer 3, and a second electrode 4 is provided on a substrate 1 with an air space a therebetween.
The film bulk acoustic resonator having such a configuration utilizes a resonant wave obtained by causing a longitudinal-mode acoustic wave, which propagates in a direction that is perpendicular to electrode surfaces of the first electrode 2 and the second electrode 4, to be reflected and resonated at a boundary surface of the piezoelectric material layer 3. However, within the piezoelectric material layer 3, a transverse-mode acoustic wave, which propagates in a direction that is parallel to the electrode surfaces, is also induced. In a case where such a transverse-mode acoustic wave is repeatedly reflected at a region in which the first electrode 2 and the second electrode 4 overlap each other, that is, an end of the resonant portion A operating as a film bulk acoustic resonator, and a standing wave is thus generated, electrical characteristics of the film bulk acoustic resonator and a band-pass filter using the film bulk acoustic resonator are significantly deteriorated.